1. Field of the Invention
The present invention relates to an electrophotographic light-receiving member that is sensitive to electromagnetic waves such as light (that is light in a broad sense meaning ultraviolet rays, visible light, infrared rays, X rays, .gamma. rays, or the like).
2. Related Background Art
In the field of image formation, a photoconductive material used to form a light-receiving layer in a light-receiving member is required to have such characteristics as high sensitivity, high SN ratio [photocurrent (Ip)/dark current (Id)], absorption spectrum compatible with the spectrum characteristic of radiated electromagnetic waves, quick photoresponse, and desired dark resistance value, no adverse affection for human beings in use, or the like. In particular, for light-receiving members integrated into electrophotographic apparatuses used as business machines in offices, the above mentioned non-polluting property is very important in use.
Photoconductive materials that are excellent in this point include hydrogenated amorphous silicon (hereafter referred to as "a-Si:H"), and its application as an electrophotographic light-receiving member is described in, for example, U.S. Pat. No. 4,265,991.
Such a light-receiving member is generally formed by heating a conductive support to 50-350.degree. C. and forming a photoconductive layer comprised of a-Si on the support using a film formation method such as a vacuum evaporation method, a sputtering method, an ion plating method, a thermal CVD method, a photo CVD method, a plasma CVD method, and the like. In particular, the plasma CVD method that decomposes a source gas by a high frequency or microwave glow discharge to form an a-Si deposited film on the support is put to practical use as a preferable method.
In addition, U.S. Pat. No. 5,382,487 proposes an electrophotographic light-receiving member comprised of a conductive support and a photoconductive layer of a-Si containing halogen atoms as a constituent (hereafter referred to as "a-Si:X"). The patent reports that incoporating 1 to 40 atomic % of halogen atoms into a-Si provides high heat resistance and good electrical and optical characteriaticsthe for a photoconductive layer of an electrophotographic light-receiving member.
In addition, Japanese Patent Application Laid-Open No. 57-115556 describes a technique for providing on a photoconductive layer composed of an amorphous material containing silicon atoms as a host, a surface barrier layer composed of a non-photoconductive amorphous material containing silicon and carbon atoms, in order to improve electrical, optical, and photoconductive characteristics such as dark resistance value, photosensitivity, photoresponse, etc. and operating environment characteristics such as humidity resistance, etc. and to also improve aging resistance for a photoconductive member having a photoconductive layer composed of an a-Si deposited film. Furthermore, U.S. Pat. No. 4,659,639 describes a technique for a photosensitive member formed by stacking a light-transmissive insulating overcoat layer containing amorphous silicon, carbon, oxygen, and fluorine, and U.S. Pat. No. 4,788,120 describes a technique for using as a surface layer an amorphous material containing as constituents silicon and carbon atoms and 41-70 atomic % of hydrogen atoms.
Japanese Patent Application Laid-Open No. 62-83470 discloses a technique for setting to 0.09 eV or less the characteristic energy of the Urbach tail of an optical absorption spectrum of a photoconductive layer of an electrophotographic photosensitive member to obtain a high quality image free of the ghost phenomenon. In particular, Japanese Patent Application Laid-Open No. 58-88115 discloses that the support side of the photoconductive layer contains a larger amount of atoms belonging to Group IIIb in the periodic table in order to improve the image quality of an amorphous silicon photosensitive member, and Japanese Patent Application Laid-Open No. 62-112166 discloses a technique for generating a carrier transport layer while maintaining the flow ratio of B.sub.2 H.sub.6 to SiH.sub.4 at 3.3.times.10.sup.-7 or more to prevent the ghost phenomenon.
In addition, to improve the image quality of an amorphous silicon photosensitive member, Japanese Patent Application Laid-Open No. 60-95551 discloses a technique that an image formation process such as charging, exposure, and development is carried out while maintaining the temperature near the surface of a photosensitive member at 30-40.degree. C. to prevent decrease in surface resistance caused by adsorption of moisture at a surface of a photosensitive member and generation of image smearing accompanying the decrease.
These techniques have improved the electrical, optical, and photoconductive characteristics and operating environment characteristics of electrophotographic light-receiving members, which has also improved the image quality.
Although the conventional electrophotographic light-receiving members having a photoconductive layer composed of a-Si-based material have each been improved in their electrical, optical, and photoconductive characteristics such as a dark resistance value, photosensitivity, photoresponse, etc. and their operating environment characteristics, aging resistance, and durability, there is still room for improvement in the overall characteristics.
In particular, since the image quality, operation speed, and durability of the electrophotographic apparatus are improved rapidly, it is necessary to further improve the electrical and photoconductive characteristics of the electrophotographic light-receiving member and to significantly improve the performance in every environment while maintaining the chargeability and sensitivity.
Since the optical exposure device, developing device, and transfer device in the electrophotographic apparatus have been improved to improve the image characteristics of the apparatus, the electrophotographic light-receiving member is also required to have more improved image characteristics than the prior art.
In these circumstances, the above conventional techniques have enabled these characteristics to be improved to some degree, but have not sufficiently improved the chargeability or image quality in some cases. In particular, to further improve the image quality of amorphous silicon based light-receiving member, it is further required to reduce the variation of the electrophotograhic characteristics due to a change in ambient temperature and optical memory such as blank memory or ghost.
For example, in the prior art, to prevent the image smearing of a photosensitive member, a heater for heating the drum is installed in a copying machine to maintain the surface temperature of the photosensitive member at about 40.degree. C. as described in Japanese Patent Application Laid-Open No. 60-95551 as mentioned above. However, in the prior art photosensitive members, the temperature dependence of chargeability resulting from the generation of pre-exposure carriers or thermally excited carriers, that is, the so-called temperature characteristic is large, so that they must be used with chargeability lower than its inherent chargeability in an actual operating environment in a copying machine. For example, when the drum is heated at about 40.degree. C., the chargeability may sometimes be lowered by about 100 V compared to that in operation at room temperature.
In addition, in the past, even during night when the copying machine is not used, the drum heater has been supplied with power to prevent image smearing from occurring by adsorption of ozone products generated by corona discharge from a charging device to the surface of the photosensitive member during night. At present, however, every effort is made to avoid the power supply to the copying machines during night in order to save resources and power. When copying is carried out under such conditions, the ambient temperature of the photosensitive member in the copying machine gradually increases to lower the chargeability, thereby sometimes causing a phenomenon that the image density varies during copying.
Furthermore, when the same manuscript is repeatedly copied, the so-called ghost phenomenon may occur in which a ghost of an image exposure during the preceding copying process appears on the image during the current copying, or the blank memory may occur in which a difference in image density is generated on a copied image due influence of the so-called blank exposure provided to the photosensitive member between every paper for toner saving during a continuous copying process. These phenomena obstruct the improvement of the image quality.
On the other hand, the recent wide spread of use of computers in offices and homes requires the electrophotograhic apparatus to be digitalized to serve not only as a copying machine as in the past but also as a facsimile or printer. A semiconductor laser or an LED that is used as an exposure light source for such a digitalized apparatus mainly uses a relatively large wavelength ranging from near infrared radiation to red visible light due to its emission strength and costs. This results in the need to improve those aspects of the electrophotographic apparatus which are not taken into account for conventional analog apparatuses using halogen light.
In particular, the use of a semiconductor laser or LED is characterized by that the relationship between the exposure and the surface potential of the photosensitive member, i.e., the so-called E-V characteristic (curve) shifts depending on temperature (temperature characteristic of sensitivity), or that the E-V characteristic (curve) becomes dull to lower its linearity (linearity of sensitivity).
That is, in a digital apparatus using a semiconductor laser or LED as an exposure light source, there has been posed a problem that when the temperature of the photosensitive member is not controlled by the drum heater, then due to the temperature characteristic of sensitivity or the lowering in the linearity of sensitivity, the ambient temperature varies the sensitivity to also vary the image density.
Furthermore, with respect to the optical memory described above, there has been posed a new problem that since the wavelength of a semiconductor laser or LED used as an exposure light source ranges from near infrared radiation to red visible light and is relatively long, and therefore since light carriers are generated in a relatively deep place relative to the surface, as compared to the conventional analog apparatus, the photocarriers are thus likely to remain to generate optical memory.
Thus, in designing an electrophotographic light-receiving member, it is necessary to improve the layer configuration of the electrophotographic light-receiving member and the chemical composition of each layer from the standpoint of overall characteristics while further improving the characteristics of the a-Si material itself so as to solve the above problems.